In 1985, about 140 MM lbs. of blowing agents (primarily CFC-11 and CFC-12) were used in the U.S. to produce all types of insulation foams. Of this total volume, about 70% or 100 MM lbs. were used to make polyurethane foam. Closed-cell polyurethane foam is the most energy efficient insulating material available, having an R value of approximately 7.2 per inch; whereas fiberglass has an R value of approximately 3.1 per inch.
Closed-cell polyurethane foams are widely used for insulation purposes in building construction and in the manufacture of energy efficient electrical appliances. In the construction industry, polyurethane (polyisocyanurate) board stock is used in roofing and siding for its insulation and load-carrying capabilities. Poured and sprayed polyurethane foams are also used in construction. Sprayed polyurethane foams are widely used for insulating large structures such as storage tanks, etc. Pour-in-place urethane foams are used, for example, in appliances such as refrigerators and freezers plus they are used in making refrigerated trucks and rail cars.
In the early 1970s, concern began to be expressed that the stratospheric ozone layer (which provides protection against penetration of the earth's atmosphere by ultraviolet radiation) was being depleted by chlorine atoms introduced to the atmosphere from the release of fully halogenated chlorofluorocarbons. These chlorofluorocarbons are widely used as propellants in aerosols, as blowing agents for foams, as refrigerants and as cleaning/drying solvent systems. Because of the great chemical stability of fully halogenated chlorofluorocarbons, according to the ozone depletion theory, these compounds do not decompose in the earth's atmosphere but reach the stratosphere where they slowly degrade liberating chlorine atoms which in turn react with the ozone.
Concern reached such a level that in 1978 the U.S. Environmental Protection Agency (EPA) placed a ban on nonessential uses of fully halogenated chlorofluorocarbons as aerosol propellants. This ban resulted in a dramatic shift in the U.S. away from chlorofluorocarbon propellants (except for exempted uses) to primarily hydrocarbon propellants. However, since the rest of the world did not join the U.S. in this aerosol ban, the net result has been to shift the uses of chlorofluorocarbons in aerosols out of the U.S., but not to permanently reduce the world-wide total chlorofluorocarbon production, as sought. In fact, in the last few years the total amount of chlorofluorocarbons manufactured has exceeded the level produced in 1978 (before the U.S. ban).
During the period of 1978-1987, much research was conducted to study the ozone depletion theory. Because of the complexity of atmospheric chemistry, many questions relating to this theory remain unanswered. However, if the theory is valid, the health risks which would result from depletion of the ozone layer are significant. This, coupled with the fact that world-wide production of chlorofluorocarbons has increased has resulted in international efforts to reduce chlorofluorocarbon use. Particularly, in Sept. 1987, the United Nations through its Environment Programme (UNEP) issued a tentative proposal calling for a 50 percent reduction in world-wide production of fully halogenated chlorofluorocarbons by the year 2000.
Because of this proposed reduction in availability of fully halogenated chlorofluorocarbons such as CFC-11 and CFC-12, alternative, more environmentally acceptable, products are urgently needed.
As early as the 1970s with the initial emergence of the ozone depletion theory, it was known that the introduction of hydrogen into previously fully halogenated chlorofluorocarbons markedly reduced the chemical stability of these compounds. Hence, these now destabilized compounds would be expected to degrade in the atmosphere and not reach the stratosphere and the ozone layer. The accompanying Table lists the ozone depletion potential for a variety of fully and partially halogenated halocarbons. Greenhouse potential data (potential for reflecting infrared radiation (heat) back to earth and thereby raising the earth's surface temperature) are also shown.
______________________________________ OZONE DEPLETION AND GREENHOUSE POTENTIALS Ozone Depletion Greenhouse Blowing Agent Potential* Potential** ______________________________________ CFC-11 (CFCl.sub.3) 1.0 0.4 CFC-12 (CF.sub.2 Cl.sub.2) 0.9 1.0 HCFC-22 (CHF.sub.2 Cl) 0.05 0.07 HCFC-123 (CF.sub.3 CHCl.sub.2) less than 0.05 less than 0.1 HCFC-124 (CF.sub.3 CHFCl) less than 0.05 less than 0.1 HFC-134a (CF.sub.3 CH.sub.2 F) 0 less than 0.1 HCFC-141b (CFCl.sub.2 CH.sub.3) less than 0.05 less than 0.1 HCFC-142b (CF.sub.2 ClCH.sub.3) less than 0.5 less than 0.2 HFC-152a (CHF.sub.2 CH.sub.3) 0 less than 0.1 ______________________________________ *Calculated relative to CFC11. **Calculated relative to CFC12.
Halocarbons such as HCFC-123, HCFC 123a and HCFC-141b are environmentally acceptable in that they theoretically have minimal effect on ozone depletion. However, these halocarbons cause cell shrinkage or collapse when used as blowing agents for closed-cell polyurethane foams. This shrinkage is particularly apt to occur with relatively flexible polymers such as those prepared from polyether polyols and when the closed-cell polyurethane foam is a low density foam, e.g., less than about 2.0 lbs./cu. ft., particularly about 1.5 lbs./cu.ft. As compared to higher density foams, the production of lower density foams generally requires a larger quantity of blowing agent and a smaller amount of polymer which results in thinner and, consequently, weaker cell walls. In addition, the halocarbon blowing agents may migrate from the cell cavities to the bulk polymer and soften or plasticize the already thin cell walls.
Finally, foam cells are most fragile just after preparation as the temperature of the foamed polyurethane returns to ambient. At the temperature at which foams are made, which normally reaches about 200.degree.-300.degree. F. because of the exothermic reaction between polyol and isocyanate, the cells contain blowing agent at one atmosphere pressure. However, after cooling to ambient temperature, foams typically contain blowing agent at less than atmospheric pressure. Since this creates a partial vacuum in the cells, they will shrink to smaller volumes if the cell walls are weak, soft or very thin. Generally, this shrinkage occurs within 72 hours of preparation and before the polymer in the cell walls fully cures and hardens. This shrinkage or collapse of foams is undesirable and results in:
(a) loss of insulation value; PA1 (b) loss of structural strength; PA1 (c) pulling away of foam from walls, e.g., walls of a refrigerator.
Such shrinkage or collapse of polyurethane insulation foam makes these halocarbons unattractive as blowing agents. It might be possible, however, to reformulate the polyurethane foam formulations to be more compatible with these halocarbons. Such a solution would require a complete study of the preparation/properties/uses of new formulations and would require a considerable period of time for development. Long term, new or modified polymer systems designed for use with these halocarbons and other more polymer-soluble blowing agents will most likely be developed; however, what is needed, as fully halogenated chlorofluorocarbon blowing agents face regulation and usage restriction, are blowing agent systems which can be used in present, essentially unmodified, polyurethane foam formulations.